Vestibular schwannoma is the most common benign tumor in the cerebellopontine angle (CPA). Management goal is to achieve optimal resection while preserving local structures, especially for CN VII (1–3). The retrosigmoid approach is the workhorse for management of CPA lesions. Most vestibular schwannomas present with intracanalicular extensions (4). In retrosigmoid approach, drilling the posterior wall of IAC is one of the key steps to dissect the intracanalicular portion of the tumor. However, for tumors located at the lateral end of IAC, “blind spots” under the microscope may impede gross total resection. With advances of techniques and technologies, the application of an endoscope can potentially solve the problem. However, the role of an endoscope during microsurgery has not been fully investigated. Here we report a retrospective single-operator series of 61 cases of vestibular schwannoma with intracanalicular extensions, where an endoscope was applied at the end of the microscopic phase to confirm the presence of remnants if tumor extends over 10 mm into IAC. Early facial nerve outcomes were satisfactory, and the post-operative course was uneventful.

Gross total resection has been associated with improved quality of life and potentially long-term control of vestibular schwannomas (3, 5, 6). Most vestibular schwannomas have intracanalicular portions. One of the most common causes of vestibular schwannoma recurrence is remnants at the fundus of IAC (6, 7). To achieve maximal resection for tumors with intracanalicular extension, care must be taken to prevent concomitant injuries to the posterior SCC, the endolymphatic duct, and a potential HJB (8, 9). Injuries to the former two structures may cause hearing loss, while injuries to the later may lead to hemorrhage and air embolisms (9). Retrosigmoid approach, a workhorse in surgical management of CPA lesions, is a versatile option in terms of preserving functions in most tumor sizes (10–13). However, the preservation rate of CN VII functions using retrosigmoid approach vary from 80 to 92% in tumor sizes of 1–2 cm to 50–76% in those > 2cm (14). Many factors, namely, surgical skills, tumor sizes, tumor extensions (intracanalicular and/or extracanalicular), cystic characteristics, and the topography of IAC may all or in part contribute to worse CN VII outcomes (15–23). Due to a relatively smaller sample size, we did not find any pre-operative CN VII dysfunctions in our sample. This would in effect diminish the impact of pre-operative HB scale on post-operative HB scale.

To reach the intracanalicular portion, drilling the posterior wall of IAC is one of the key steps in retrosigmoid approach (8, 24). The surgical nuance here is to ensure maximal exposure of the posterior wall of IAC with protection of neighboring structures. In a healthy individual, the shape of porus acusticus is oval or elliptical (25). The average length of IAC ranges from 5.5 to 12.3 mm, mean diameter 4 mm, according to the present literature (26, 27). To date, the standard of optimal drilling range has not been established. At least three parameters are required to define a drilling range: the length of lateral drilling, the length of the drilled posterior wall of IAC, and the drilling angle. The length of lateral drilling was proposed by some experts to be 10 ± 2 mm (28). The length of drilled posterior wall of IAC ranged from 5 to 10 mm, according to the present literature (28–30). Using frameless navigation on 10 cadaveric heads, Pillai et al. concluded the angle of drilling as 43.3 ± 6.0˚ (24). In our experience, we made a “flexible rule”, in which the length of lateral drilling was approximately 12–14 mm, drilling angle 43˚ and the length of drilled posterior wall of IAC 8 mm ( Figure 2 ). After about 8 mm of the posterior wall of IAC was drilled, there were still blind spots at the lateral end of IAC under microscopic view. If tumor remnants were located there, the risk of nerve injuries under “blind” manipulations would increase. Therefore, the application of an endoscope should theoretically minimize the risks of nerve injuries caused by “blind dissection”.

Many reports of endoscopic usage in vestibular schwannomas and/or other CPA lesions were seen in literature (24, 31–33). The purely endoscopic approach requires fewer brain retractions, and the advantage of increased visualization at IAC by navigating through narrow corridors would minimize the risks of complications (34). However, the usefulness of purely endoscopic approach for vestibular schwannomas is still in debate (34–36). Having an assistant to hold the endoscope with both hands may overcome the single-handed technique by the operator, but the dynamic movement of the endoscope requires constant searching for a pivoting point for balance. The bulky manipulation by two hands and the time-consuming nature of endoscopic surgery may compromise visual stabilization. A pneumatic endoscope holder is surely more versatile but less flexible than freehand manipulation. In addition, navigating solely by endoscope is prone to disorientation and concomitant injuries to the vital and complex neurovasculature of CPA (37). Therefore, microscopic resection is still the mainstay of treating vestibular schwannomas.

Using a neuroendoscope after microscopic phase is the optimal tactic to circumvent these problems. The surgery should always start with microscopic dissection, which is familiar and less time-consuming for many neurosurgeons. For tumors with less extension into IAC, microscopic dissection would be adequate. While in tumors with ≥10 mm extension into IAC, we would opt for an extra endoscopic phase. The optimal visualization ( Figure 6 ) brought by the neuroendoscope would identify potential tumor remnants and facilitate gross total resection using angled instruments (35, 36). If a routinely performed 8 mm of drilling is not enough for more laterally located tumors, an extra 2 mm could be drilled under endoscopic visualization to manage more laterally located tumor remnants.

In practice, neuroendoscopy involves a significant learning curve that demands extensive training. During the surgery, the assistant needs to be familiar with not only the procedure, but also the preferences of the operating surgeon. It is also important for the assistant to irrigate the neuroendoscope to avoid debris accumulation on the tip and thermal injuries to nerves. Above all, in the hands of an experienced neurosurgical team, endoscope-assisted microsurgery of vestibular schwannomas should be considered as a safe and feasible treatment modality, especially for tumors with lateral extension over 10 mm into IAC. Due to a limited sample size, these findings are only preliminary. Further studies are needed to validate the cutoff line of the switching-to-endoscope tactic during microsurgical resection of vestibular schwannomas via the retrosigmoid approach.

In Grade B vestibular schwannomas, after maximal microsurgical removal, endoscopic evaluation of the intracanalicular portion revealed residual tumors in 17% of the patients. Hence endoscopic evaluation of the potential intracanalicular remnants for tumor extending over 10 mm within IAC is recommended.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by the Institutional Review Board of Shanghai General Hospital. Written informed consent to participate in this study was provided by the participants’ legal guardian/next of kin.

YB, YN, DG, and ML conceived of and directed the study. QinZ, QiaZ, and JT provided clinical information and analysis the data. JL and FS participated in data acquisition. HL was responsible for data acquisition and figure productions. JY and YL reviewed the manuscript. All authors contributed to the article and approved the submitted version.

This work was supported by the National Natural Science Foundation of China (No. 81902529), the Health Technology Project of Pudong New District Health Committee (PW2020B-2), the Scientific Research Funding Project of Shanghai University of Medicine & Health Sciences Affiliated Zhoupu Hospital (ZPXM-2019A-01), and the Cross Research Fund of Medicine and Engineering of Shanghai Jiao Tong University (Nos. YG2017QN29 and YG2021QN90).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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